This invention relates generally to illumination systems for display devices and particularly to such a system for a liquid crystal display device capable of producing color television pictures.
Liquid crystal display devices used to produce color displays typically require backlighting to achieve the image brightness required for viewing in day light ambient conditions. Existing illumination systems frequently employ a diffuser just before the liquid crystal array in order to smoothe the spatial distribution of light from the lamp. Such a diffuser causes the light entering the liquid crystal array to be spread over a wide range of angles. However, it is known that the contrast of the display can be improved if the angular range of the entering light rays in one azimuth is confined to 15 degrees or less from the perpendicular to the array. There is no restriction for ray components having the orthogonal azimuthal orientation. One existing system uses parabolic reflectors with light sources at the focii of the reflectors to provide partially collimated light. Various lens systems are employed to further control the angles over which the light is incident on the liquid crystal cells. Another scheme for back illuminating the liquid crystal cells of a display device includes a solid light conduit which is illuminated at one end, and which has prisms along a second surface to release the light in a controlled manner to the liquid crystal cells.
In a liquid crystal display device capable of producing a full spectrum of colors, each pixel of the display is composed of a triad of pixel elements. Each of the pixel elements is a liquid crystal cell which transmits one of the primary light colors. The liquid crystal cells themselves do not produce color and accordingly each cell must be associated with a filter of the appropriate light transmission capability. Also, the light provided to the liquid crystal cells must be polarized because of the intrinsic functional characteristics of the liquid crystal cells. A substantial percentage of the illumination, as much as 85%, is lost to the filters and polarizers. Accordingly, the illumination efficiency of liquid crystal display devices is very low and the illumination source is called upon to produce a significantly higher intensity of light than is actually needed to illuminate the display screen. As a result, a very large percentage of the available energy of the display device is consumed by the illumination source. This is a particularly posing disadvantage in portable display devices because it results in a substantial reduction in the lifetime of the power source and, thus, in the viewing time of the display.
For these reasons there is a need for a more efficient illumination system for display devices. The present invention fulfills this long felt need.